There are many references to methods of preparing foamed polyester resin compositions, usually dealing with foaming and crosslinking as separate operations. Either a foam is prepared and then crosslinked without destroying it, or the resin is crosslinked while gas is being released into it. In either of the approaches the gas may be provided by a variety of means.
Azocarboxylates are very efficient blowing agents releasing about three times as much gas as for instance azodicarbonamide, the most widely used chemical blowing agent. No method has been known wherein azocarboxylates are employed as the sole blowing agents for the expansion of polyester resin, i.e., there is no commercially practical method to use such a system. This deficiency is overcome by the instant invention.
U.S. Pat. No. 3,095,387 June 25, 1963 discloses use of azodicarboxylate for the expansion of liquid polysulfide rubber.
U.S. Pat. No. 3,111,496 Nov. 19, 1963 teaches the use of azodicarboxylates for the expansion of organoplastic at a temperature of 148.degree. C. or higher.
U.S. Pat. No. 3,993,609 Nov. 23, 1976 discloses the use of certain azo-compounds other than azodicarboxylates for the expansion of polyester resins.
Japanese Patent Application No. 79/166918 teaches the expansion of polyester foam using a combination of certain hydrazides with azodicarboxylic acid salts.